Typewriters have traditionally used type levers or spherical type elements. More recently typewriters have begun using the daisy wheel print element which requires a hammer to be fired and impacted against the individual petals of the print wheel or daisy wheel to cause the print font on the petal of the wheel to impact against the printing ribbon and paper.
One drawback to the utilization of a daisy wheel type printer in a typewriter is that a typist needs print line visibility to observe what has just been typed. The close positioning of the type wheel to the print point diminishes print line visibility very substantially. Not only does the type wheel itself tend to block print line visibility, but also the ballistic hammer typically found in many daisy wheel printers must be positioned such that its trajectory intersects the plane of the print wheel at the print point, thereby further reducing print line visibility.
Alternative hammers utilized to impact the daisy wheel print petals occasionally use a lever acting against the hammer structure. While this is advantageous from the standpoint of reducing the hammer assembly size, the hammer itself is still positioned to substantially reduce visibility.
A lever hammer typically is activated by a magnet acting on one end which then forces the opposite end of lever into engagement with the print element petals. This type hammer may be subject to variations in magnetic field strength, poor magnetic efficiency, and high tolerance requirements. This type hammer relies upon the magnetic field strength to force the printing end of the hammer into engagement with the print petal.
The characteristic of the magnetic field forcing the hammer element into contact with and holding the petal onto the platen during the printing stroke is also characteristic of some of the ballistic hammer designs. In this type of hammer, the impact is dependent upon the voltage and the current at the point of impact.